Palaeogeography and dynamics of the deltaic wetland of Save River, Mozambique

Palaeogeography, Palaeoclimatology, Palaeoecology 489 (2018) 64–73

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Palaeogeography and dynamics of the deltaic wetland of Save River, Mozambique

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Elídio A. Massuanganhea,b,⁎, Annika Berntssonb,c, Jan Risbergb, Lars-Ove Westerbergb,c, Marie Christianssonb,c, Frank Preusserd, Stefan Bjursäterb, Mussa Achimoa a

Department of Geology, Faculty of Sciences, Eduardo Mondlane University, CP. 257 Maputo, Mozambique Department of Physical Geography, Stockholm University, S-10691 Stockholm, Sweden c Bolin Centre for Climate Research, Stockholm University, S-10691 Stockholm, Sweden d Institute of Earth and Environmental Sciences, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany b

A R T I C L E I N F O

A B S T R A C T

Keywords: Delta progradation Mangrove Save River delta Biogeomorphology

Many mangrove wetland systems in deltaic environments are negatively affected by massive sedimentation from river inflows. In this paper we use the example of the Save River delta to assess the palaeogeographic distribution of mangrove wetlands and to analyze their dynamics. To track past occurrences of mangrove wetlands in the study area we have integrated sedimentological data with siliceous microfossil analysis combined with AMS radiocarbon and OSL dating. The results show a fine-grained deposit with an approximate thickness of 2 m, present at different sampling sites. In the upper deltaic plain, the deposit is interbedded between sand layers, while in the lower deltaic plain the deposit occupies the uppermost stratigraphic position. In most of the sampling sites the deposit shows a succession with brackish-marine diatoms at the bottom of the sequence while the upper part shows only scattered occurrences. Based on sedimentological and microfossil characteristics we have interpreted the layer to represent a mangrove wetland deposit. The development of the deposit in the study area is suggested to have been initiated around 3100 cal. yr BP, induced by sea-level rise. Thereafter, the development followed the combined effect of a sea-level fall and delta progradation processes. In some areas, particularly in the proximal part of the delta, the mangrove deposit has developed progressively on top of the delta-front. From around 1300 years ago (OSL) onwards, massive alluvial sedimentation impacted the mangrove ecosystem. However, the retreat of mangrove wetland coincided with a regional fall of sea level. At the edges of the alluvial deposit, the current mangrove ecosystem has reclaimed the habitat in some sectors where gully erosion has exposed the once extinct mangrove habitat.

1. Introduction Mangrove forest has been noted in stratigraphic records at least since the Paleogene/Neogene (Willumsen et al., 2014; Roberts et al., 2017). They are important habitats, today occurring between the latitudes 25° S and 25° N (Giri et al., 2011), providing plentiful natural resources to coastal dwelling communities and shelter from floods and storms (Massuanganhe et al., 2015). However, our knowledge of mangrove wetland ecosystems is still in its incipient stage when it comes to comprehending the continuous changes that they undergo. Several studies have aimed to investigate and understand the functionality of these complicated ecosystems (e.g. Lugo and Snedaker, 1974; Burchett et al., 1984; Smith et al., 1991; Ball and Pidsley, 1995; Lee, 1999). Complementary studies are, however, necessary to face the challenges posed by climate change and by the high exploitation of

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these ecosystems (Nicholls et al., 1999; Nicholls, 2004; Gedan et al., 2011). The increasing degradation rate that mangrove wetland systems experience today (Woodroffe and Grime, 1999; Datta et al., 2012; Pabari et al., 2013; Alongi, 2014; Santos et al., 2014; Chaudhuri et al., 2015) has raised awareness among coastal managers and decision makers about the importance of implementing adaptation and mitigation measures (Maes et al., 2012; Cools et al., 2013). Unfortunately, some of the implemented management initiatives are limited to short term solutions, often without concern for the natural dynamics of the landscape (Das et al., 2012; Miloshis and Fairfield, 2015). Any management practice that inhibits the continuous dynamics of the landscape, which is crucial for mangrove ecosystems, is not sustainable (Syvitski and Saito, 2007; Syvitski et al., 2013; Giri et al., 2015). To ameliorate this situation, more examples on how the mangrove wetland systems have reacted to climate and weather conditions in the past are

Corresponding author. E-mail address: [email protected] (E.A. Massuanganhe).

http://dx.doi.org/10.1016/j.palaeo.2017.09.021 Received 4 January 2017; Received in revised form 9 September 2017; Accepted 18 September 2017 Available online 20 September 2017 0031-0182/ © 2017 Elsevier B.V. All rights reserved.

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10 m above the mean sea level (m a.m.s.l.). In general, the topographic heights in the upper deltaic plain are higher in the sectors adjacent to the river and decrease gradually from the proximal part of the delta to the distal part. In the lower deltaic plain the maximum topographic height is ~3 m a.m.s.l. Stabilized dunes with heights reaching ~ 8 m a.m.s.l. occur within the lower deltaic plain. Distributary river channels and tidal channels run through the lower deltaic plain being influenced by semi-diurnal macrotidal conditions. The transition area between the upper and lower deltaic plains is characterized by a number of streams. Tide amplitude in the delta is in the order of 4 m. Geologically, the study area is composed of alluvium with grain size fractions ranging from clay to gravel which accumulated during the Holocene (DNG, 2006). Detailed records of sea-level fluctuation during the Holocene are not available for the study area, but records from South Africa show evidence of fluctuations between +3.5 m (ca. 5000 cal. yr BP) and −2 m (ca. 3100 cal. yr BP) (Ramsay, 1995) relative to present-day sea level. A study on palaeoenvironmental change in Macassa Bay, located 150 km south of the study area, suggests that sea-level was higher than at present between 6600 and 6300 cal. yr BP, and between 4600 and 1000 cal. yr BP (Norström et al., 2012).

essential (Thom, 1967; Woodroffe, 1992; Reinhardt et al., 2010). A number of palaeoenvironmental studies have recorded long-term development of mangrove wetland systems and have associated their dynamics with changes in relative sea level (e.g. Woodroffe et al., 1985; Ellison and Stoddart, 1991; Fujimoto et al., 1996). This development coincides with the evolution process of river deltas, which has taken place from the early Holocene onwards (Woodroffe et al., 1985; Bird et al., 2007; Tamura et al., 2009). During the same time period, ecological conditions within the mangrove wetland systems have also been influenced by variable climate and weather conditions, as shown by palaeoenvironmental reconstructions based on pollen and siliceous microfossils (Behling et al., 2001; Behling et al., 2004; Prebble et al., 2005; Punwong et al., 2012; Srivastava and Farooqui, 2013; França et al., 2015). The sensitivity of diatoms to variations in salinity was early recognized by researchers and has been used to reconstruct coastal palaeoenvironments (Risberg, 1986; Juggins, 1992; Haldorsen et al., 2004), including mangrove wetlands (Ross et al., 2001; Soeprobowati et al., 2012). As mangrove wetland systems develop under brackish water conditions, the degree of salinity has been used to identify these environments. Mangrove wetland deposits are generally characterized by fine-grained minerogenic sediments rich in organic matter and peat (Woodroffe, 1992; Augustinus, 1995; Woodroffe, 2013). Like in many other regions worldwide, mangrove dieback has been reported from eastern Africa and has been associated with human pressure and high sediment accumulation rates (Erftemeijer and Hamerlynck, 2005; Bandeira et al., 2009; M'rabu et al., 2012). These high rates are in most cases related to floods and storm surges (M'rabu et al., 2012), however, the sedimentary imprints of such weather events are likely enhanced by erosion caused by human activities (Neumann et al., 2010; Giguet-Covex et al., 2011; Giguet-Covex et al., 2012; Reinwarth et al., 2013). In this study we use the example of the Save River delta to analyse the pattern of changes on mangrove wetland systems within an evolution perspective. Using an integrative sedimentological, geomorphological and palaeoecological approach we aim to discuss the current paradigm, according to which sedimentation is viewed as a potential environmental problem for the mangrove wetlands. Therefore, the main purpose of this study is to identify and reconstruct the mangrove wetland and its dynamics. A secondary purpose is investigate the applicability of diatom analysis for reconstructing past occurrences of mangrove habitats in the Save River delta.

3. Methods Initially we undertook a preliminary investigation where morphological characteristics of the study area were interpreted based on a SPOT satellite image from 2007 (Fig. 1). Subsequently, fieldwork was undertaken to describe the morphological and sedimentological characteristics of the study area and to control the interpretations based on the SPOT satellite image. During fieldwork samples were collected for further analyses in laboratory. Samples, preliminarily interpreted to represent marine-brackish clays, were selected for siliceous microfossil analysis, mainly diatoms. The purpose of these analyses was to achieve a qualitative assessment of the interpretation of a marine-brackish depositional environment. Fourteen samples were collected from eight sampling sites to give information on age relations. 3.1. Fieldwork In the field we performed an overall description of the geomorphology of the study area. To implement the work, we selected eight sampling sites (P1–P8, Supplementary Table 1) for detailed descriptions regarding lithostratigraphy and for systematic collection of samples for laboratory analysis. The sampling sites P1–P3 are located on the riverbank within the upper deltaic plain. The sampling sites P4–P8 are located within the transition area between the upper deltaic plain and the lower deltaic plain. The sedimentological descriptions and grain size estimations were performed in the field. On the riverbank, descriptions were undertaken in cleaned vertical sections and each layer was identified using textural characteristics, colour and sedimentary structures. We have used a soil auger, gouge auger and piston corer samplers to retrieve sediments from varying depths. For the fine-grained units we subsampled at 10 cm interval for microfossil analysis. Simultaneously, we collected samples for radiocarbon and optically stimulated luminescence (OSL) dating. Ten samples of dateable material (Supplementary Table 2) were collected and sent to Poznan Laboratory for AMS radiocarbon dating. In most of the sampling sites the dateable material was scarce. As an alternative, in-situ roots were considered and subject to further discussion. OSL samples were collected by driving opaque sampling tubes into sandy layers located immediately below and above a distinct fine-grained layer. Lithological descriptions and subsampling for microfossil analysis were performed in 2014. The positions of the radiocarbon dates from 2012 were correlated with the lithostratigraphy established in 2014. The data from each sampling site were summarized graphically in sedimentary logs and integrated with the laboratory results.

2. Study area The study area is located in the deltaic plain of the Save River, in south-eastern Africa (Fig. 1). The delta is developed on the coastal plain of Mozambique and its wetland area extends further north throughout a sector of the coast dominated by rivers. The mangrove wetland in this sector is part of a large areal extent of mangrove in the region of eastern Africa (Mitra et al., 2005; Giri et al., 2011; Daru et al., 2013; Chaudhuri et al., 2015). This region is also cyclically affected by tropical cyclones (Reason and Keibel, 2004; Mavume et al., 2009) that have caused damage on coastal ecosystems (Bosire et al., 2012; Pabari et al., 2013). The south-eastern region of Africa, including the study area, is dominated by SE winds that induce northward longshore currents. The Save River is one of the largest rivers in the region with a catchment area of approximately 102,000 km2, mainly located in Zimbabwe. In Mozambique the river runs through a narrowed basin for approximately 300 km and ends by two distributary channels (Matasse Channel and Macau Channel) in the Indian Ocean (Fig. 1). During the rainy season (between October and March), the catchment area is episodically supplied with excessive water that floods the study area contributing sediment load to the deltaic plain, particularly for the sectors adjacent to the river and its distributary channels. The upper deltaic plain (Fig. 1) is represented by an alluvial deposit adjacent to the main river channel, with topographic heights ranging from 3 m to 65

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Fig. 1. The Save River delta in Mozambique. The Satellite Probatoire d'Observation de la Terre (SPOT) image shows the landscape attributes of the study area. The dashed line indicates the boundary between the upper and lower deltaic plains. P1–P8 mark positions of sampling sites.

3.2. OSL dating

3.3. Siliceous microfossil analysis

Subsampled sediment for luminescence measurements was sieved through 200–250 μm mesh size and treated with HCl, H2O2 and Na4P2O4 to remove carbonates, organic and clay particles, respectively. Quartz grains were enriched though density separation using LST heavy liquid (2.7 g cm− 3 and 2.58 g cm− 3), followed by etching in HF (40%, 1 h) and HCl treatment. Sample dose rates were determined through high-resolution gamma spectroscopy measurements of radionuclides on an adjacent batch of sediment. Luminescence measurements of samples P1 and P2 (2 m depth) were conducted with a Lexsyg research reader by Freiberg Instruments (Richter et al., 2013) at Stockholm University, whereas samples P2 (5 m depth) and P3 were measured at the University of Bern with a Risø DA-20 TL/OSL reader. A modified SARprotocol (Murray and Wintle, 2003) was modified to implement an initial step of infrared stimulation (IRSL at 50 °C) at the onset of the protocol to assess quartz purity, followed by 10 s preheating (225 °C) and subsequently 60 s blue LED stimulation at 125 °C for measurement of the natural signal. Dose equivalences (De) were integrated from the first 0.4 s (channels 1–4) of the emitted OSL signals. For age calculations, weighted De averages were obtained with the Central Age Model (CAM) (Galbraith et al., 1999) from aliquots passing internal recycling criteria of ≤ 10% (Murray and Wintle, 2003). In addition to the CAM, mean De values were extracted using the Minimum Age Model (MAM) of Galbraith et al. (1999), applying an expected overdispersion of 10%.

Subsamples for diatom analysis were collected in field contiguously at intervals of 10 cm thickness from open sections (P1, P2 and P3) or from cores (P6 and P8). Carbonates were removed by adding 10% HCl and organics were removed by boiling the samples in 17–35% H2O2 until the reaction ceased (Battarbee, 1986). Clay and sand sized particles were removed by repeated decanting based on settling times in water according to Stoke's law. When the liquid seemed clear, a weak NH3 solution was added for disaggregation and the settling procedure was repeated until the liquid phase was clear again. Excessive water was removed and the remaining silt-sized fraction was mounted in Naphrax®. Diatoms were identified under light microscope (Leica DMLS and Zeiss Axiophot) at × 1000 and × 1260 magnification using immersion oil. Identifications and grouping according to salinity tolerance were based on Cleve-Euler (1951), Foged (1975), Gasse (1986) Witkowski et al. (2000), Levkov (2009) and Krammer and LangeBertalot (1991, 2008a, 2008b, 2010). Groups used were brackishmarine taxa, halophilic taxa (preferring raised salinity compared to freshwater), indifferent taxa (occurring in both fresh and brackish water), freshwater taxa, aerophilic taxa and unknown taxa (where identification was impossible due to poor preservation or bad positioning on the slide). Diatom valves, diatom fragments, chrysophyte cysts and sponge spiculae were counted. Phytoliths were not counted as main focus was on salinity changes. Diatom fragments were counted for the river bank sections to give indications of fragmentation/ 66

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Fig. 2. Lithostratigraphic representation of the described sections of seven sampling sites with the respective summary of diatom counts and percentages. Note that diatoms are given as counts, except for P2 where they are given as percentages. + symbols represent each occurrence of a diatom taxon when they are less than five. Zonations are based on lithological units except for P2 where they are based on Constrained Cluster Analysis. * Percentages of diatom fragments, chrysophyte cysts and sponge spiculae were calculated based on the diatom basic sum. Dates within brackets at P2 have been transferred from a previous description of a similar section located ca. 30 m to the west. OSL dates are presented in years ago.

details of the AMS radiocarbon dated samples. Observed and identified diatoms are listed and grouped in Supplementary Table 3. Phytoliths were observed on slides from all sampling sites. Below is a summary for each sampling site. Bright and stable OSL emissions, as well as low IRSL signals (< 50 cts), confirms suitability of the sample material for dating applications as well as a satisfactory purity of the quartz. The apparent OSL ages of samples P1 and P2 (Supplementary Table 4) are, however, represented by relatively large overdispersion values compared to what is typically observed in alluvial deposits. De distributions of most of the samples show indication for partial bleaching, which would lead to age overestimation. Clear subpopulations can visually be identified from the De distributions which have also been approximated by the CAM (Galbraith et al., 1999). One sample (at site P3) appears well-bleached as indicated by an overdispersion value just below 10%. Acknowledging the presence of partial bleaching in most of the samples, we henceforth refer to the application of ages derived from the MAM for interpreting the development of the study area.

taphonomy. Complete diatom valves were counted as one unit and half valves, containing the central area, as half a unit. When central areas could be identified they were counted as one unit if they were more than the halves of the same taxon together in that subsample. In such cases the halves were disregarded as to avoid double counting of individual specimens. Diagrams were constructed in Tilia 1.7.16 (Grimm, 1991-2011). For the main stratigraphy in P2, taxa were presented as percentages of total diatom composition, and Constrained Cluster Analysis (CONISS) was performed as a basis for zonation. Percentages of chrysophyte cysts, sponge spiculae and diatom fragments were calculated in relation to the diatom basic sum. Owing to low diatom concentrations, results are given as counts for the stratigraphies at P1, P3, P6 and P8. Taxa with ≤ 5 valves in one subsample are represented by ‘+’ in the diagrams to highlight their presence. For these stratigraphies zonation lines were added based on lithological units to facilitate comparisons between the different sampling sites. 4. Results Figs. 2 and 3 show the bio- and lithostratigraphic characteristics of the sampling sites. At P1, P2 and P3 a fine-grained layer, composed of clay and silty clay, is interbedded between coarse-grained material. At the other sampling sites a fine-grained layer occupies the uppermost parts of the sections. The thickness of this fine-grained layer varies from 150 to 200 cm among the sampling sites in general, except at P1 where the thickness is up to 350 cm. Supplementary Table 2 summarizes the

4.1. Sampling site P1 Sedimentologically, the lower part of the sedimentary log at P1 consists of very coarse sand. Immediately above, the coarse-grained layer is covered by a fine-grained layer ranging between 625 and 260 cm depth (Fig. 2). The lower part of the fine-grained layer (between 625 and 510 cm depth), consists of fine sand with planar laminations 67

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Fig. 3. Typical transversal cross-sections of the gullies exemplified at P7. The cross sections show the lithostratigraphy and the positions where the samples were collected. The Fig. also shows the geomorphological history in which (a) the three layers are horizontally stratified, (b) gully erosion has removed the uppermost layer, and (c) erosion has exposed the two lowermost layers to supratidal conditions.

sand shows a lens of silty sand at approximately 430 cm depth. The uppermost layers of silt and clay (370–240 cm) are characterized by an increasing content of organic matter. An important aspect is the distribution of the radiocarbon dates and the OSL dates, which are not in chronological order. The oldest radiocarbon date indicates an age of ca. 2050 cal. yr BP (Poz-53660) in the interval 470–480 cm, while the youngest date ca. 250 years (OSL) is stratigraphically below, from 500 to 505 cm (Fig. 2). The radiocarbon dates indicate ages between 2050 cal. yr BP (Poz-53660) and ca. 750 cal. yr BP (Poz-53659), while both OSL dates indicate lower age of ca. 250 years. Brackish-marine diatoms dominate this riverbank succession, particularly in the lower part (Fig. 2). However, indifferent and freshwater diatoms are present in all analysed samples, and together with aerophilic taxa they show some increase upwards in the sequence. All samples studied contain diatoms. The diagram has been divided into five diatom assemblage zones (DAZ 1-5). The percentages of chrysophyte cysts and sponge spiculae are higher in the three uppermost zones than in the two lowermost. Also the percentage of diatom fragments in relation to whole and half diatom valves are higher in the upper zones. The zone boundaries are generally in agreement with lithological units. DAZ 1 (490–450 cm depth): This zone is within a sediment unit that

interstratified with silt (Fig. 2, P1 zone 1). This zone is characterized by a change in diatom concentration rather than in proportions of ecological groups. There is a gradual decrease in the abundance of all taxa up to 460 cm depth. The diatoms are mainly brackish-marine or indifferent but also freshwater taxa occur. In zone 2, consisting of dark clay and dispersed carbonate concretions < 3 cm in diameter, no diatoms were encountered in the subsamples. Radiocarbon dates suggest ages of ca. 3100 cal. yr BP (Poz-67397) at ~600 cm depth and ca. 1000 cal. yr BP (Poz-60019) at ~320 cm depth. Between 260 and 0 m, there is a sand layer interstratified with silt, typical of alluvial deposits. The OSL-date from the lower part of this layer indicates an age of ca.1300 years. 4.2. Sampling site P2 The lithology at P2 consists of coarse brown sand in the lowermost part of the section (Fig. 2), overlain by grey and clean silty sand (490–440 cm), dark-grey clay (440–370 cm), silt (370–320 cm) and clay (320–240 cm). The clay is overlain by sand with planar and trough cross-stratifications. The boundary at 490 cm, between the lowermost sand and the silty sand, shows convolute bedding structures and the lamination in the silty sand is deformed. The lowermost clay layer that overlies the silty 68

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while brackish-marine, indifferent and freshwater diatoms are present.

consists of silty sand. Brackish-marine diatoms dominate with ~55%, mainly Diploneis interrupta, D. pseudovalis and Hantzschia distinctepunctata. Indifferent (e.g. Amphora copulata) and freshwater (e.g. Gyrosigma obtusatum) diatoms make up ~25% in some samples. Aerophilic diatoms occur with up to 6%. DAZ 2 (450–440 cm depth): The silty sand in this interval belongs to the same lithological unit as the previous zone. The present zone comprises only one sample but stands out, having a clear brackishmarine signal (~ 90%) with only low percentages of other taxa. Typical taxa are Coscinodiscus radiatus v. parvus, D. interrupta, D. pseudovalis, Nitzschia granulata, Melosira moniliformis and Paralia sulcata. DAZ 3 (440–370 cm depth): Lithologically, this zone corresponds to a clay unit. Brackish-marine diatoms contribute up to 31–75% of the total assemblage, mainly D. interrupta supplemented by H. distinctepunctata. Freshwater taxa contributes up to 25% being represented by e.g. Aulacoseira granulata, Cymbella meulleri and Eunotia spp. Indifferent taxa occur with up to 20% (e.g. A. copulata, Epithemia adnata and Rhopalodia gibba). Aerophilic taxa occur with up to 9% and halophilic taxa with up to 3%. DAZ 4 (370-320 cm depth): Lithologically this zone corresponds to a silt unit. It marks the transition to a more mixed diatom flora. In all but the lowermost sample indifferent and freshwater taxa outweigh brackish-marine taxa. Brackish-marine taxa constitute 22-43% (mainly D. interrupta and H. distinctepunctata), indifferent taxa 25-34% (mainly A. copulata, E. adnata and R. gibba) and freshwater taxa 6-25%. DAZ 5 (320–240 cm depth): This zone corresponds to a clay unit. Considering the diatom content it is similar to the previous zone, but with a slight shift towards more freshwater and aerophilic taxa (less indifferent A. copulata and more freshwater Navicula cuspidata, A. grunulata and D. elliptica, and aerophilic Hantzschia amphioxys). Brackishmarine taxa contribute with 16–50% (mainly D. interrupta and H. distinctepunctata), indifferent taxa with 13–42% (mainly E. adnata), freshwater taxa with 11–29%, aerophilic taxa with up to 20% and halophilic taxa with up to 6%. Additionally, the sand dated for OSL (at 500–505 cm depth), was briefly scanned for diatoms. It was found to contain some diatoms, of similar composition as the overlying sample in the clay unit. Brackishmarine D. interrupta, D. pseudovalis and H. distinctepunctata were most common, but also indifferent, freshwater and aerophilic taxa occurred.

4.4. Sampling site P4 The lower part of the P4 section consists of bright-grey unconsolidated and medium sand up to 160 cm depth. Between 160 cm depth and the ground surface, the section consists of dark-grey clay rich in organic matter, mainly in the upper part. The colour of the clay layer varies from dark-grey to brownish. The radiocarbon date at 340–347 cm depth is calibrated to ca. 950 cal. yr BP (Poz-60018). Sediments in the lithostratigraphic section were not analysed for siliceous microfossils. 4.5. Sampling site P5 The basal unit of P5 consists of light-grey and loose sand up to 170 cm depth, with no visible internal structures. Between the top of the sand layer and the ground surface, the section consists of clay, with interstratified medium to fine sand between 140 cm and 100 cm depth. The upper section of the clay unit is rich in organic matter. From 70 cm depth to the surface the organic matter is oxidized and reddish-brown. The bottom of the clay layer is dated to ca. 450 cal. yr BP. Roots/plant remains further up in the core generated slightly younger ages (Supplementary Table 2, Fig. 2). This section was not analysed for siliceous microfossils. 4.6. Sampling site P6 P6 is composed of medium sand from the base to 130 cm depth. The sand is unconsolidated, characterized by dark-grey colour without visible internal structures. Between 130 cm and 30 cm depth, the section is composed of dark-grey clay rich in organic matter grading to reddish grey in the upper part of the layer. From 30 cm depth to the surface the section is composed of silty sand with a yellowish grey colour. Diatoms were observed in 11 of 29 samples analysed (Fig. 2). Most of the samples from the upper part of the sand unit, and just above it from 250 to 125 cm depth (~ zone 1), have a brackish-marine diatom flora dominated by D. interrupta, P. sulcata and Terpsinoë americana. Diatoms were absent in most samples from the clay unit (~ zone 2). The uppermost sample, however, contains aerophilic diatoms, mainly N. mutica and P. borealis.

4.3. Sampling site P3 The lower part of the section P3 consists of yellowish-brown coarse sand with trough cross-stratification structures. At 360 cm depth, the sand is overlain by 15 cm dark grey silt. In the upper surface of the silt layer the colour is reddish-brown. From this surface up to 200 cm depth there is a medium grey sand layer interstratified by 30 cm of silty clay layer between 330 cm and 300 cm depth. The sand in the layer is homogeneous and without clear stratification. The interstratified silty clay layer is dark-grey with less content of organic matter. Between 200 cm and 100 cm depth the section shows a silty clay layer commonly characterized by an interstratification pattern between dark- and bright-grey colour. The topmost 100 cm layer consists of medium to coarse sand with trough cross-stratification structures that incorporate mud pebbles. The OSL date from the lowermost sand in this section points to an age of ca. 200 years. In this riverbank section only the fine-grained units and one sample from the underlying sand were analysed for siliceous microfossils (Fig. 2). All analysed samples contained diatoms. Chrysophyte cysts and sponge spiculae also occurred in several of the samples. There is a clear brackish-marine signal in analysed samples below ~300 cm depth (zones 1, 2, 3), with mainly D. caffra, D. interrupta, D. pseudovalis, H. distinctepunctata, Hyalodiscus sp., Nitzschia granulata and P. sulcata as the dominating taxa. In the uppermost clay unit (200–100 cm depth, zone 4) there is a more mixed signal with aerophilic diatoms such as Hantzschia amphioxys, Navicula mutica and N. paramutica dominating,

4.7. Sampling site P7 Lithologically, the lowermost part of the section P7 consists of darkgrey clay. Superimposed on this clay layer is 35 cm of fine silty sand with light-greyish to yellow colour. Above this silty sand layer is a 80 cm thick dark-grey clay layer. In sample P7a (Fig. 3), which is taken from the deposit now cut by the river, few diatoms were encountered, e.g. the aerophilic H. amphioxys. The sample also contains scattered diatom fragments and is rich in phytoliths, suggesting a strong terrestrial influence. In P7b, recovered from the deposit below the present mangrove level, no diatoms and only few phytoliths were encountered. However, despite the lack of diatoms, the sample contains several sponge spiculae, which suggests that the sediment was deposited in water or in peat wetlands (Struyf and Conley, 2009; Maldonado et al., 2010). Sample P7c from the flank of the present tidal channel is characterized by mainly brackish-marine diatoms Diploneis suborbicularis, D. smithii and Gyrosigma wansbeckii. 4.8. Sampling site P8 The lowermost part of the section P8 consists of medium sand to 210 cm depth. Between 210 and 15 cm the core is composed of compacted grey clay. Between 15 cm and the surface, the core is composed 69

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fauna. The gully erosion and the stratigraphic position of the clay layers indicate that the depressions have previously been filled with alluvial material, which may have caused extinction of mangrove forest in the area (Fig. 3). Therefore, the current mangrove trees that develop in this area represent a new generation of mangrove, established as result of the exposure of the old habitat together with water exchange from the tides, one of the prerequisites for mangrove development. Although many studies have shown mangrove regeneration after disturbance (Paling et al., 2008), little is known about the landward reclamation of mangrove areas, such as the one demonstrated in this study. The discussed example can be adopted as a tool for recovering mangrove ecosystem for those places with similar palaeogeographic settings, or where mangrove ecosystems are heavily impacted by sedimentation. In P2 (Fig. 2) the fully analysed 250 cm sequence (490–240 cm) contained variable but comparatively high concentrations of diatoms, with a dominance of brackish-marine Diploneis interrupta throughout the sequence. The generally higher counts in P2, as compared with the other studied sections, suggest that the sediment was deposited in a more sheltered basin with better preservation conditions for diatoms. There is a slight trend towards more freshwater influence. The strongest marine influence is at 445 cm depth with e.g. Coscinodiscus spp. and Paralia sulcata. From 370 cm depth, there is a shift towards higher percentages of freshwater and indifferent taxa such as Aulacoseira granulata and Epithemia adnata. However, there is a constant mix with marine-brackish, indifferent and freshwater taxa. From ~440 cm depth, aerophilic taxa are more common and the proportion of diatom fragments increase, indicating somewhat stronger terrestrial influence and more erosive conditions. From this part of the stratigraphy there are two radiocarbon dates (Poz-53658 and Poz-53659) with reverse ages, suggesting that the dated material consists of reworked terrestrial organic matter (cf. Stanley and Hait, 2000). The OSL dates suggest much lower ages and thereby higher sediment accumulation rates, although the lower of the two OSL-dates is probably erroneous, as it indicates an unlikely high accumulation rate (20 mm/year). P2 may represent a quasi-protected delta sub-basin that has been acting as a recipient for freshwater, sediment and terrestrial plant material brought by the river, and for marine-brackish water brought by the tides. From our results, we observe a general shoreward decrease of ages but with some reversed dates at P2. These results suggest a shoreward mangrove wetland succession in agreement with the delta progradation (Fig. 4). However, in more detailed stratigraphic correlation attempt, the fine-grained layer may not necessarily show perfect continuity, because of the influence of laterally shifting incisive channels during development. In addition, the surface between the fine-grained layer and the superimposing sand is not geometrically continuous because of the typical wedge pattern that characterize this surface (cf. Mellere and Steel, 1995; Petter and Steel, 2006). Within this perspective, we interpret that the mangrove wetland system may have started developing in the study area around 3100 cal. yr BP. As the mangrove prograded shorewards, sedimentation of alluvium in the proximal part of the delta may have caused progressive dieback of mangrove as evidenced by the overlaying alluvial sand on fine-grained sediments, particularly at P1, P2 and P3 (Fig. 2). We suggest that the mangrove dieback was caused by massive sedimentation, and that this commenced around 1300 years ago (as dated by OSL at P1, 220 cm depth). Ages in the mangrove deposit generally vary along the section in agreement with the vertical sedimentation that occurs in wetland systems (Woodroffe, 1992; Walsh and Nittrouer, 2004; Rao, 2005; Woodroffe et al., 2015) except as discussed for P2. Therefore, some of the alarming sedimentation in mangrove ecosystems, specifically in deltaic environments may be linked to long-term evolution.

of silty sand with light-grey colour. Five out of 23 scanned samples contain diatoms. The two lowermost samples in the clay unit contain brackish-marine diatoms only, mainly Hyalodiscus sp., and sponge spiculae. The two uppermost samples contain almost exclusively aerophilic diatoms such as H. amphioxys, N. mutica and Pinnularia borealis. 5. Discussion We have identified a fine-grained layer in the stratigraphy at each sampling site in this study. This layer is generally grey and composed of silt and clay and is in some places rich in organic matter. The textural composition of this sediment layer, dominated by silt and clay, is common in mangrove wetland deposits and in other environments where water velocity is reduced (cf. Augustinus, 1995; Ellison, 2008). In addition, the results of the microfossil analysis show brackish-marine species in the lower parts of the stratigraphic columns suggesting brackish subaquatic conditions, which are typical of a mangrove wetland system. Overall, the diatom assemblages from the Save River delta samples typically show a mix of ecological groups: freshwater taxa from the river, marine taxa transported inland by the tidal flows, indifferent taxa tolerant of shifts in salinity and osmotic pressure, and aerophilic taxa that are more drought resistant and therefore survive in wetlands and semi-terrestrial environments. In this study, we have observed a general scarcity of diatoms in the upper parts of the fine-grained layer for most of the sampling sites, with the exception of P2. It has been noted in other studies that preservation of diatoms in coastal sediments, including mangrove systems, is negatively affected by combined physiological and chemical stresses (Sherrod et al., 1989; Ross et al., 2001). Therefore, because of low occurrences, focus is put on species observations, not on diatom assemblages, i.e. a qualitative rather than a quantitative approach. 5.1. Interpretation of the combined sedimentary and diatom record For the lower deltaic plain, the fine-grained layer occupies the uppermost part of the stratigraphy. The current mangrove wetland is close to the sampling sites P4, P5, P6 and P8, showing similar sedimentary packages characterized by approximately 2 m of a fine-grained sediments superimposed on sand (cf. Massuanganhe et al., 2015). This leads us to correlate the fine-grained layer with the current mangrove habitat. The fine-grained units at P6 and P8 correlate well with each other also based on the diatom successions, i.e. marine-brackish signals in the lower parts and aerophilic taxa at the top. In addition, P5, where the fine-grained layer occupies a similar position, is located within a supratidal plain with scattered mangrove trees. Similarly, the cores at P6 and P8 have been collected in a supratidal flat where the uppermost part is covered by terrestrial grasses. We infer that fluvial processes have supplied the silty sand from aeolian origin that occupies the uppermost 15–30 cm at these sites during floods (Fig. 2). This interpretation is supported by the diatom stratigraphy that shows predominance of aerophilic species in the upper 30 cm of the sections at both sites (P6 and P8). Aerophilic species may have been successively brought from the previously deposited terrestrial environments, including the dunes, and distributed over fine-grained layer of mangrove deposit. Fig. 3 synthetizes part of the geomorphologic history that has taken place in the study area. The cross-section illustrates the typical transversal profile of the gullies and streams that characterize the transition area between the lower deltaic plain and the upper deltaic plain. The streams originated from rills developing into gullies. In the depression area there are still some remaining erosional stacks with the same lithostratigraphic characteristics as the banks of the depression. Looking at the lithological characteristics represented in Fig. 3, we argue that runoff and was responsible for shaping most of the gullies. The gullies are under influence of tidal currents, which has promoted the reactivation and development of mangrove trees and the associated

5.2. Taphonomy and representativeness of the diatom record The scarcity of diatom frustules in many of the subsamples, e.g. P6, P7 and P8, may not reflect true depositional conditions, but may be a 70

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Fig. 4. Idealized palaeogeographic distribution of mangrove wetland in a longitudinal cross section of Save River delta.

result of dissolution of biogenic silica. For example, samples P7a and P7b (Fig. 3) were collected from clayey sediment deposits that were expected to contain diatoms, as in the more recently deposited sample P7c, but few were encountered. However, P7a and most of the other diatom poor samples do contain biogenic silica, such as phytoliths and, irregularly, sponge spiculae. The latter confirm that the sediments were deposited in water or wetland (cf. Struyf and Conley, 2008; Maldonado et al., 2010). The more recently deposited sediment at P7c, from the reclaimed mangrove area, was rich in diatoms, suggesting there may be issues of poor preservation of diatoms in some of the older deposits. A possible explanation for this is uptake of dissolved silica by semiaquatic and terrestrial vegetation (Struyf and Conley, 2008; Epstein, 2009; Das et al., 2014). This causes undersaturation of silica in the soil solution, which may result in corrosion of the biogenic silica in diatoms. High temperatures and high bacterial activity further increases dissolution of biogenic silica (Struyf et al., 2005), and dissolved silica can also be drained from the sediment by tidal floods (Jacobs et al., 2013). These processes may explain the generally low diatom concentrations in our samples, especially in regularly drained layers that have been affected by the roots of vegetation such as sedges and mangrove plants and by bacterial activity. The concentrations of biogenic silica are commonly higher in the uppermost sediments (~ 10 cm) of both freshwater and tidal marshes and decreases downward (Struyf et al., 2005). The difference in specific surface between diatoms on the one hand and phytoliths and sponge spiculae on the other may lead to differential dissolution (Miller et al., 1990; Maldonado et al., 2010), which is a possible explanation why the latter are present while the former are not. The implication for our full record is that we have possibly lost part of the salinity signal. At P2, the conditions for diatom preservation suggested to have been more favourable due to the faster burial times, which led to shorter exposure time to taphonomic processes. Heavily silicified diatoms are also more likely to survive the highly dynamic conditions in the estuarine environment with resuspension of sediment and tidal action and wind stress. The most common taxon in the studied samples was Diploneis interrupta (Fig. 5), which is a highly silicified, benthic diatom that occurs mainly in brackish water. Its heavily silicified structure may cause better post-deposition preservation than other species, possibly resulting in relative overrepresentation.

Fig. 5. Diploneis interrupta, the most common diatom species preserved in the analysed samples.

reported from the area (Fonseca et al., 2014), there are no records of isostatic tilting, uplift and/or subsidence. We therefore assume that any late Holocene sea-level variation is mainly caused by global eustacy. A fall in mean sea level is a critical factor for mangrove retreat, but also sea-level rise is a cause for mangrove dieback (Ellison and Stoddart, 1991; Gilman et al., 2007). Under conditions of sea-level rise, sedimentation can compensate for the effects (Krauss et al., 2014). Looking to our case study, it is challenging to ascertain whether the late Holocene sea-level exclusively has influenced the development of the mangrove. Within the study area, however, the sedimentation of the finegrained layer may have started ca. 3100 cal. yr BP, the approximate time when the regional sea-level started to rise from a lowstand (Ramsay, 1995). Therefore, we interpret that the fine-grained layer at P1 may have initiated its deposition from sea-level lowstand (at ca. 3100 cal yr BP) and followed the rising sea-level up to ca. 1300 years ago. The subsequent sea-level fall registered during the last millennium (Jaritz et al., 1977; Ramsay, 1995; Norström et al., 2012) may have favoured the shoreward shift of the mangrove wetland, when simultaneously the coarse-grained fluvial sediments covered gradually the proximal sector of the delta. The shift in this scenario is possible when accompanied by shoreward decrease of the bathymetry under nearshore conditions. Despite the scenario of sea-level fluctuations, sedimentary sequences of palaeo-mangrove deposit coincide with the models of delta progradation (Ta et al., 2002; Gani, 2005). These models show a continuous process in which the wetland sediment is being deposited shoreward on top of the delta-front (Fig. 4). This interpretation is in agreement with the ages observed in this deposit towards the shore.

5.3. Sea-level changes vs. delta progradation The interception of the fine-grained layer observed throughout the deltaic deposit suggests that the mangrove habitat is representatively distributed in the delta. Similar studies on the palaeogeography of mangroves point out sea-level as one of the variables that control the development of deltaic wetlands (e.g. Woodroffe and Grindrod, 1991; Behling et al., 2001; Behling et al., 2004; Srivastava and Farooqui, 2013; Woodroffe et al., 2015). Although earthquakes have been 71

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Comparing the interlayered fine-grained layers from P1, P2 and P3 (Fig. 3) with the architecture discussed by Gani (2005), we find similarities, particularly in how the fine-grained layers overlie the sediments from the delta-front and are overlain by alluvial material. Therefore, we suggest that the combined effect of sea-level change and natural progradation of the delta were the main decisive factors determining the spatio-temporal development of the wetland deposit.

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6. Conclusions In this study, we have identified a fine-grained sedimentary layer with typical characteristics of a mangrove wetland deposit. The layer is quasi-continuous throughout the deltaic plain. In the upper deltaic plain the mangrove deposit is covered by alluvial sediments while in the lower deltaic plain the deposit occupies the uppermost stratigraphic position, where the modern mangrove forest flourishes. The mangrove wetland deposit may have started its development around 3100 cal. yr BP, in a process we suggest was dominated by combination of sea-level change and shoreward delta progradation. Through thisprocess, vertical accretion has continued until the present in the lower deltaic plain. At the proximal sector of the delta the mangrove wetland has lasted ca. 1800 years before the alluvial sedimentation starts. The sedimentation of the alluvial deposit registered in the upper deltaic plain has been taking place until the present, as observed in the transition zone between the upper deltaic plain and the lower deltaic plain where thin layers of very fine sand and alluvial clay cover the modern mangrove sedimentary deposits. In the sectors where alluvial sedimentation has occurred within the intertidal range, there are interlayers of finegrained sediments were deposited between the pulses of fluvial deposition. The main finding of this study is evidence of landward mangrove reclamation in a sector of the coast previously covered by alluvial sediments showing a mechanism in which geomorphology may control the mangrove ecosystem. In light of this finding, there are possibilities for reactivating mangrove ecosystem in places where sedimentation has caused massive dieback. In addition, this study demonstrates that part of the alarming sedimentation reported in mangrove habitat worldwide may be related to natural morphodynamic processes. It is concluded also that diatom analysis is a valuable complementary method in reconstructing mangrove habitats in the Save River delta. Taphonomic processes may occasionally affect the species distribution, thus influencing biostratigraphic interpretations. Acknowledgments This research work was supported by SIDA (SIDA Decision No. 2011-002102) under a bilateral cooperation programme between Sweden and Mozambique, and the programme was executed by Stockholm University (Sweden) and Eduardo Mondlane University (Mozambique). The authors are thankful to Mr. Albino Chidala from the administration of Govuro District for assisting in the fieldwork. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.palaeo.2017.09.021. References Alongi, D.M., 2014. Mangrove forests of timor-leste: ecology, degradation and vulnerability to climate change. In: Mangrove Ecosystems of Asia. Springer, pp. 199–212. Augustinus, P.G.E.F., 1995. Geomorphology and sedimentology of mangroves. Geomorphol. Sedimentol. Estuar. 53, 333–357. Ball, M.C., Pidsley, S.M., 1995. Growth responses to salinity in relation to distribution of two mangrove species, Sonneratia alba and S. lanceolata, in Northern Australia. Funct. Ecol. 9, 77–85. Bandeira, S.O., Macamo, C.C.F., Kairo, J.G., Amade, F., Jiddawi, N., Paula, J., 2009. Evaluation of mangrove structure and condition in two trans-boundary areas in the Western Indian Ocean. Aquat. Conserv. Mar. Freshwat. Ecosyst. 19, S46–S55.

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